Higher Average Temperatures Linked To Chikungunya Risk In India

by Sahana Ghosh, Mongabay

The risk of transmission of chikungunya in India increases with elevated average temperatures, a climate-based modeling study has said.

Other studies also support the notion that there is a range of temperatures where risk of disease is highest and temperatures higher and lower have less risk.

The infections of chikungunya in India are due to availability of vectors (mosquito) and optimum temperature conditions that influence faster chikungunya transmission.

The risk of transmission of chikungunya in India increases with a steady spike in average temperatures in the south Asian nation, a climate-based modeling study has said.

Researchers suggest the susceptible regions for the disease may shift towards non-endemic areas in India due to population movement and availability of suitable eco-climatic conditions for the mosquito vector.

Chikungunya, a mosquito-borne viral disease, is transmitted to humans by the bites of infected female mosquitoes, most commonly, the Aedes aegypti and Aedes albopictus species which can also transmit other mosquito-borne viruses, including dengue.

Like other vector-borne diseases, chikungunya disease transmission involves host, vector (mosquito) and pathogen (virus) and is influenced by the climatic factors such as temperature and rainfall.

The infections of chikungunya in India are due to availability of vectors (mosquito) and optimum temperature conditions that influence faster chikungunya transmission, said study lead author M. Srinivasa Rao, scientist at the CSIR-Indian Institute of Chemical Technology, Hyderabad.

"Temperatures are steadily increasing and this has triggered faster parasite development and mosquito life cycle. So the numbers of transmission are gradually increasing. Our model helps in public health preparedness and vector management operations in predicted risk zones of chikungunya well in advance," Rao told Mongabay India.

The study shows that chikungunya spreads between 20 and 34 degree Celsius but the peak transmission occurs at 29 C. No transmission takes place below 17 C and above 34 C.

Moreover, a small variation in climatic factors has a huge impact on vector-borne diseases such as malaria, Zika, dengue and chikungunya, he said. Rainfall creates breeding habitats for vectors and temperature has a major role for both mosquitos, virus development and transmission.

The researchers developed a model for chikungunya that maps the temperature-dependent transmission potential of the disease using biting rate (the average blood meal frequency), vector to host and host to vector transmission of chikungunya virus, extrinsic incubation period (EIP) and mortality rate of the mosquito vectors. EIP is the time it takes for the chikungunya virus (in this case) to develop within a mosquito, and become transmissible..

For the study, conducted as part of the Environmental Information System (ENVIS) project of the Ministry of Environment, Forest and Climate Change (MoEFCC), researchers analysed datasets spanning 68 years (1948 to 2016), which revealed that during 1982–2016 many states reported gradual increase in risk of chikungunya transmission when compared with the 1948–1981 period.

Chikungunya cases were reported more frequently during the 1982–2016 period in southern, northern and western parts of India. During this period, mean temperatures gradually increased in south India when compared with data of 1948–1981 time scale.

"At those temperatures, the Aedes aegypti and Aedes albopictus species of mosquito, displayed higher biting rate, lower extrinsic incubation period and lower mortality rate. The incubation period shortens as the temperature increases," said Rao.

The outbreaks of chikungunya in India mostly coincide with the monsoon and early post-monsoon periods. During monsoon period rainfall provides ample number of breeding habitats for both Aedes albopictus and Aedes aegypti, temperature enhances the growth of vector, influences mosquito density and biting activity.

Spread varies across India

The study also mapped the variation in chikungunya spread across Indian states.

Based on the disease spread data from 2006 to 2016, it is seen that the highest average incidence rates (per million population) were observed from West Bengal, Goa, Delhi, Karnataka, Kerala and Puducherry.

"Whereas our model predicts that Gujarat, Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Telangana, Odisha and West Bengal are states where chikungunya has chances of spreading rapidly once infection is initiated. This is because temperatures are quite high and in the conducive range for the spread," Rao said.

"There are a few chikungunya cases reported in northeastern states but the intensity is very low," Rao said.

In India, most of the coastal areas (east and west coast of India) and southern region show the environmental suitability for chikungunya transmission. Similarly, central region and parts of north and northeast region show low suitability for chikungunya due to prevailing low temperatures.

The disease was first described during an outbreak in southern Tanzania in 1952. In India, the outbreak of chikungunya was first reported in Kolkata (then Calcutta) in 1963. Subsequent outbreaks and sporadic cases were reported from Tamil Nadu, Andhra Pradesh and Maharashtra in 1964–65 and 1973 (Maharashtra only).

A resurgence of the chikungunya virus and its rapid global spread has been observed since 2004. The disease re-emerged in India in 2005, after a gap of 32 years, causing massive outbreaks in some states and circulating thereafter. From 2006 to 2016 a total of 383,260 chikungunya cases were reported in India. The highest incidence rate (84.6 per million population) was reported in the year 2008, the study found.

During the year 2010 and 2016, major outbreaks of chikungunya occurred in Delhi. Over 12,000 cases of chikungunya were reported in 2016 in Delhi. "This outbreak was highly associated with warm temperature and during these periods strong El Nino conditions were prevailing," Rao said.

In India's neighbourhood

In another study, researcher Joseph L. Servadio from Brown University School of Public Health, looked at extreme temperature and precipitation, using the highest monthly average temperature and precipitation in a year, to predict risk of mosquito-borne diseases in south Asia and southeast Asia.

Using outbreak data from the Global Infectious Disease and Epidemiology Online Network (GIDEON) for the period 1980 to 2009, the analysis spanned Bangladesh, Cambodia, India, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Thailand and Vietnam.

"We combined diseases, but most cases were chikungunya or dengue. We found that, in this broader region that does include India, when the highest monthly average is around 34 C, diseases were most likely to be seen in that year," Servadio told Mongabay-India.

The team's findings support the notion that there is a range of temperatures where risk of disease is highest and temperatures higher and lower have less risk.

"It's also worth noting that mosquitoes don't adhere to national boundaries, so as long as the climates are hospitable to mosquitoes in nearby or neighbouring countries, there is the possibility of diseases spreading across borders. While I don't have a good example for South and Southeast Asia, this has been seen in the Americas with West Nile Virus and Zika," said Servadio.

Servadio's study supports the theory of shift: while the susceptible region will expand toward the poles, regions closest to the equator will be come too warm for disease transmission and see a decline in mosquito-borne diseases.

"This would occur because, as temperatures become too hot, the life cycles of mosquitoes shorten to where the diseases cannot adequately incubate in time. Some laboratory studies have shown various optimal ranges of temperature for mosquito longevity and speed of disease incubation, so while some regions will become warmer and more suitable for disease risk, others will become too warm and less at risk of disease," Servadio added.